If _evaluate_cumulative_extfn is supplied, this cumulative sum is evaluated where the next_value/evaluate sequence is combined into a single _evaluate_cumulative_extfn call for each row within each partition.
select b, my_sum(a) over (partition by b rows between unbounded preceding and current row) from t order by b
_start_extnfn(cntxt) _reset_extfn(cntxt) _evaluate_cumulative_extfn(cntxt, args) -- input a=1 returns 1 _evaluate_cumulative_extfn(cntxt, args) -- input a=2 returns 3 _evaluate_cumulative_extfn(cntxt, args) -- input a=3 returns 6 _reset_extfn(cntxt) _evaluate_cumulative_extfn(cntxt, args) -- input a=4 returns 4 _evaluate_cumulative_extfn(cntxt, args) -- input a=5 returns 9 _evaluate_cumulative_extfn(cntxt, args) -- input a=6 returns 15 _finish_extfn(cntxt)